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[PATCH 5/7] [BZ #18572] arm: Disable lazy initialization of tlsdesc entries
- From: Szabolcs Nagy <szabolcs dot nagy at arm dot com>
- To: GNU C Library <libc-alpha at sourceware dot org>
- Cc: nd at arm dot com, Joseph Myers <joseph at codesourcery dot com>
- Date: Tue, 24 Oct 2017 15:31:08 +0100
- Subject: [PATCH 5/7] [BZ #18572] arm: Disable lazy initialization of tlsdesc entries
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2017-10-23 Szabolcs Nagy <szabolcs.nagy@arm.com>
[BZ #18572]
* sysdeps/arm/dl-machine.h (elf_machine_lazy_rel): Do symbol binding
non-lazily for R_ARM_TLS_DESC.
>From 9732943ec024f2ea66f978cd4196ec6a71497358 Mon Sep 17 00:00:00 2001
From: Szabolcs Nagy <szabolcs.nagy@arm.com>
Date: Fri, 20 Oct 2017 17:35:12 +0100
Subject: [PATCH 5/7] [BZ #18572] arm: Disable lazy initialization of tlsdesc
entries
Follow up to
https://sourceware.org/ml/libc-alpha/2015-11/msg00272.html
Always do tls descriptor initialization at load time during relocation
processing (as if DF_BIND_NOW were set for the binary) to avoid barriers
at every tls access. This patch mimics bind-now semantics in the lazy
relocation code of the arm target (elf_machine_lazy_rel).
Ideally the static linker should be updated too to not emit tlsdesc
relocs in DT_REL*, so elf_machine_lazy_rel is not called on them at all.
* sysdeps/arm/dl-machine.h (elf_machine_lazy_rel): Do symbol binding
non-lazily for R_ARM_TLS_DESC.
---
sysdeps/arm/dl-machine.h | 24 +++++++++++++++---------
1 file changed, 15 insertions(+), 9 deletions(-)
diff --git a/sysdeps/arm/dl-machine.h b/sysdeps/arm/dl-machine.h
index bf5f5d205c..c59386f515 100644
--- a/sysdeps/arm/dl-machine.h
+++ b/sysdeps/arm/dl-machine.h
@@ -669,15 +669,21 @@ elf_machine_lazy_rel (struct link_map *map,
}
else if (__builtin_expect (r_type == R_ARM_TLS_DESC, 1))
{
- struct tlsdesc volatile *td =
- (struct tlsdesc volatile *)reloc_addr;
-
- /* The linker must have given us the parameter we need in the
- first GOT entry, and left the second one empty. We fill the
- latter with the resolver address. */
- assert (td->entry == 0);
- td->entry = (void*)(D_PTR (map, l_info[ADDRIDX (DT_TLSDESC_PLT)])
- + map->l_addr);
+ const Elf_Symndx symndx = ELFW (R_SYM) (reloc->r_info);
+ const ElfW (Sym) *symtab = (const void *)D_PTR (map, l_info[DT_SYMTAB]);
+ const ElfW (Sym) *sym = &symtab[symndx];
+ const struct r_found_version *version = NULL;
+
+ if (map->l_info[VERSYMIDX (DT_VERSYM)] != NULL)
+ {
+ const ElfW (Half) *vernum =
+ (const void *)D_PTR (map, l_info[VERSYMIDX (DT_VERSYM)]);
+ version = &map->l_versions[vernum[symndx] & 0x7fff];
+ }
+
+ /* Always initialize TLS descriptors completely, because lazy
+ initialization requires synchronization at every TLS access. */
+ elf_machine_rel (map, reloc, sym, version, reloc_addr, skip_ifunc);
}
else
_dl_reloc_bad_type (map, r_type, 1);
--
2.11.0